Golf balls are made in a variety of constructions and compositions. Generally, a core is surrounded by a cover, with at least one intermediate layer optionally disposed there between. Examples of conventional golf ball materials range from balata to polybutadiene, ionomer resins, polyurethanes, and/or polyureas. Typically, outer layers are formed about the spherical outer surface of an inner golf ball component via compression molding, casting, or injection molding.
Golf ball manufacturers continuously experiment with constructions and material formulations in order to target and improve aerodynamic and/or inertial properties and achieve desired feel without sacrificing durability or aesthetics. In this regard, simpler and more cost effective ways are sought for preserving golf ball color appearance, which can deteriorate, for example, when a white golf ball is exposed to UV light on the course and UV degradation ensues. Such UV degradation becomes visibly apparent as golf ball yellowing/browning, which is not appealing to players who equate discoloration with lower-quality and inferior golf balls.
Golf ball finishing coats, primer coatings, and cover layers have been known to need protection from exposure to UV light. Finishing coatings are often clear coats, so that any underlying primer coating or cover layer could also be directly exposed to UV light on the course. Regardless, inner surfaces are also sometimes partially exposed to UV light when nicks or abrasions occur in the golf ball surface when a club face (e.g. an iron) strikes the golf ball.
Certain golf ball materials such as polyurethane-based compositions are particularly vulnerable to UV degradation. Manufacturers have encountered the tendency of elastomers to react with molecular oxygen in a degradation process called “autoxidation.” This degradation process results in undesirable changes, such as product discoloration and loss of physical properties. Autoxidation may be initiated by heat (thermo-oxidative degradation), high energy radiation (photodegradation), mechanical stress, catalyst residues, or through reaction with other impurities. However, photodegradation by ultraviolet (“UV”) radiation is believed to be the most damaging of these autoxidation mechanisms. Thermo-oxidation and photodegradation processes are initiated with the formation of free radicals. These free radicals react rapidly with oxygen to from peroxy radicals. These peroxy radicals may further react with the polymer chains leading to the formation of hydroperoxides. On exposure to additional heat or light, hydroperoxides decompose to yield more radicals that can reinitiate the degradation process.
UV absorbers protect against photodegradation by “competing” with the polymer for absorption of ultraviolet light. An ideal UV absorber should be very light stable and should have broad, intense absorption over the UV range from about 290 nm to 400 nm. Antioxidants, on the other hand, interrupt the degradation process in different ways according to their structure. The major classifications of antioxidants are primary antioxidants and secondary antioxidants. Primary antioxidants, such as sterically hindered phenols, react rapidly with peroxy radicals (ROO) to break the degradation cycle. Secondary antioxidants, such as arylamines, are more reactive toward oxygen-centered radicals than are hindered phenols. The secondary antioxidants react with hydroperoxide (ROOH) to yield non-radical, non-reactive products, and are frequently called hydroperoxide decomposers.
The color instability caused by both thermo-oxidative degradation and photodegradation typically results in a “yellowing” or “browning” of the polyurethane material, an undesirable characteristic for urethane compositions are to be used in the covers of golf balls, which are generally white.
Initially, this problem was addressed by applying at least one layer of “paint” comprising a clear and/or pigmented topcoat material about the cover material. But repeated blows to the golf ball surface with a golf club causes scuffing or paint removal tending to result in exposing the cover material to harmful UV rays during play, ultimately resulting in undesirable “yellowing” and/or “browning” of the cover material. And apart from degradation of the cover material due to direct UV radiation exposure, degradation still occurred over a long time period and the resultant discoloration tended to “bleed” through the paint layer, also discoloring the golf ball cover. This long-felt problem in the golf ball art led golf ball manufacturers to formulate UV absorbers and stabilizers directly into the cover resin material for improved color stability upon prolonged exposure to UV light—either in addition to or in lieu of the paint layer.
However, incorporating UV absorbers and stabilizers directly into the entire cover resin material in concentrations as high as 10 wt % can represent a significant unnecessary manufacturing cost given that the UV absorbers/stabilizers are really only needed in those surface areas or regions of the layer that are exposed to UV light. And additional costs can be incurred from difficulties associated with incorporating the additives into the entire batch. Meanwhile, there is a potential for adverse effect on physical properties such as loss of shear resistance or poor tensile from UV absorbers and stabilizers being added directly into the entire batch.
Thus, there is a need for golf balls and methods of making golf balls wherein UV resistant and color stable compositions may be provided onto only those golf ball surfaces that are subject to UV light exposure rather than being added into the entire layer formulation. Such golf balls and methods would restrict protection against UV degradation to the outermost region of the cover where it is needed, thereby reducing golf ball manufacturing costs and better preserving physical properties. The present inventive golf balls and methods of making same address and solve this need.